The measurement of large loads, both forces and moments, with little uncertainty is desired in many applications. One such instance is in the measurement of the loads encountered by a scale model in wind tunnel testing, where errors and uncertainties are magnified by making full scale correlations. Typically, the instruments used for measuring these loads, known as balances, are of the following two general types; strain gaged and/or weigh beam. Of the first type, there are two common variations, those using flexural elements, and those using load cells. Regardless of which variation or combination is used, the accuracy, repeatability, and resolution are all limited by the use of strain gages and the associated output voltage reading accuracy. Intrinsic to the strain gage transducer is a need to design for stress levels which yield adequate strain at the location of strain gage installation. These stress levels often result in fatigue of the flexural elements and require frequent calibrations to ensure that the strain gages themselves are indeed securely bonded. Weigh beam balances utilize a series of fulcrums and levers to reduce the applied loads to magnitudes which can be handled by electro-mechanical force restoring coils and/or precision load cells. While stress levels can be appreciably reduced from those of a strain gage balance, weigh beam balances are generally more flexible. This flexibility results in the translation and rotation of the balance calibration center, and with that, another uncertainty is introduced. Additionally, weigh beam balances require larger volumes to accommodate the load reducing levers.
Prior art patent literature includes German Patent (patentscrift) No. 138,704 showing a scale utilizing a laser interferometry instrument for the measurement of applied weights utilizing a clamped wire for measuring forces in the downward direction.
In contrast, the present instrument is a precision, multi-piece, unidirectional scale capable of measuring both tensile and compressive (bi-directional) loads without regard to gravitational direction.
In further contrast, the present instrument has a single piece load path thereby eliminating hysteresis on a macro level and confining it to the hysteresis experienced on a molecular level, a quantity much smaller than experienced in multi-piece construction instruments.
In yet further contrast with German Patent (patentschrift) No. 138,704, linkages in which alignment is critical are not utilized; rather, deflection is confined to a specific region as that of a beam in bending, and an amplification arm is used to magnify small deflections and to isolate the measurement to the center of bending, a characteristic yielding improved system accuracy.
Further patent literature includes German Patent (patentschrifi) No. 143,956 of the interferometric type utilizing a Z-shaped, double flexure element in FIG. 12E; however, not utilizing an amplification arm for magnifying small deflections.